GIMP Arrow Designer

Make a selection where the corners delimit the two ends of the
arrow. Then run Filters->Render->Arrow designer...

The dialog lets you change which direction the arrow points within
the selection box -- for instance, use NE to have the arrow point
"northeast" -- as well as the size and angle of the arrowhead.
The color and line width will use the current foreground color and brush.
Arrow designer monitors the selection, so you can adjust its shape
and location in real time using GIMP's Rect Select tool..

Get it here: arrowdesigner-0.5.py.
Install it to your GIMP personal plug-ins folder: ~/.gimp-2.6/plug-ins
on Linux, or whatever GIMP lists in the Preferences window under
Folders / Plug-ins category. On Linux and Mac, make sure it's
executable (chmod +x arrowdesigner.py).